I have 2 furnaces, one in the garage and one in the attic. These are both spaces with unconditioned air, so they get very cold in the winter. The contractor who helped me configure my garage furnace told me that he rarely recommends condensing (high-efficiency) furnaces in unconditioned spaces as the water condensate they discharge can freeze, and then the furnace may not turn out until it's thawed. Another issue in unconditioned spaces is placing a humidifier. My original garage furnace was attached to a humidifier that froze and broke.

It's now autumn and my furnaces are heating the house significantly. As a result, they're sapping all the humidity from the air in the house and we're feeling very dry.

Is there a way to install a large or central humidifier that will not freeze? Does it need to be installed in the unconditioned spaces with the furnaces, or can it be installed elsewhere in the system?

1 Answer
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It does not need to be installed in the unconditioned space near the furnace. It should be in the duct that feeds all the others. Look for a powered humidifier. It will need a 120V outlet, water supply, water drain and it will need low voltage wires connecting it to the humidistat and the furnace. If the duct that feeds all the branch ducts is in an unconditioned space then you may have an issue.